Thermal interface having improved high-temperature operating range

ABSTRACT

A cryostat includes a thermal interface located at a first end and a cryocooler located at a second end. A thermal link has a thermal connector thermally linking the thermal interface and the cryocooler below a critical temperature. The thermal link is separated from one of the thermal interface and the cryocooler above the critical temperature. A method of operating the cryostat is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No. 61/223,896, filed on Jul. 8, 2009, which is incorporated by reference as if fully set forth.

BACKGROUND

There are many applications in which it is desirable to provide a thermal interface that is capable of exposing a sample holder to a wide range of temperatures in a controlled fashion. It is often desirable for the exposure temperatures for the sample holder to range from cryogenic temperatures (less than about 120 K) to very high temperatures (e.g., greater than 800 K).

Existing thermal interfaces comprise a cryocooler connected to the thermal interface through a thermal link and a heat source embedded in the thermal interface. It is known to use a variable conductance element (having high conductivity at low temperatures and low conductivity at high temperatures) as part of the thermal link. Use of a variable conductance element, such as a sapphire rod, reduces conduction of heat from the thermal interface to the cryocooler at high temperatures. This is important because damage to the cryocooler will result if the temperature of the cryocooler cold tip exceeds its maximum operating temperature, which is typically about 325 K. In order to enhance thermal isolation of the thermal interface, it is common to provide a radiation shield that envelops the thermal interface and a vacuum shroud that surrounds the radiation shield. A typical operating cycle consists of cooling the sample holder to cryogenic temperatures using the cryocooler, then heating the sample holder to very high temperatures (e.g., 700 K) using the heat source.

In some applications, it is desirable to heat the sample holder to temperatures above 800 K (e.g., 1000K). Unfortunately, existing thermal links are unable to provide sufficient thermal insulation to operate the thermal interface at temperatures exceeding 800 K, while maintaining the cold tip below its maximum operating temperature. Accordingly, there is a need for an improved thermal link that will enable the thermal interface to operate at temperatures well-above 800 K.

BRIEF SUMMARY OF THE INVENTION

To be completed upon approval of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the appended drawing figures wherein like numerals denote like elements.

FIG. 1 is a perspective view showing a first embodiment of a non-optical cryostat, in which the vacuum shroud and radiation shield, which are opaque, are represented in this figure as being transparent so that components located therein are visible;

FIG. 2A is a perspective view of a first embodiment of the thermal interface and thermal link portions of the cryostat shown in FIG. 1, shown with a ceramic sleeve attached thereto;

FIG. 2B is a perspective view of the first embodiment of the thermal interface and thermal link portions of the cryostat shown in FIG. 1, shown with the ceramic sleeve removed;

FIG. 3A is a sectional view, taken along line 3-3 of FIG. 2B, showing the position of the thermal link when the thermal interface is operating at a low temperature;

FIG. 3B is a sectional view, taken along line 3-3 of FIG. 2B, showing the position of the thermal link when the thermal interface is operating at a high temperature;

FIG. 4 is a graph comparing exemplary temperature performance of the cryostat shown in FIG. 1 to the prior art;

FIG. 5 is a graph showing temperature performance data of the cryostat shown in FIG. 1;

FIG. 6A is an exploded view of a second embodiment of the thermal interface and thermal link portions of the cryostat shown in FIG. 1;

FIG. 6B is a sectional view, taken along the same line as FIGS. 3A and 3B, of the second embodiment of the thermal interface and thermal link portions of the cryostat shown in FIG. 1; and

FIG. 7 is a sectional view, taken along the same line as FIGS. 3A and 3B, of a third embodiment of the thermal interface and thermal link portions of the cryostat shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. It is understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention, as set forth in the appended claims.

To aid in describing the invention, directional terms are used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional definitions are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features.

FIG. 1 shows an exemplary embodiment of a non-optical cryostat 10. The cryostat 10 includes a thermal interface 12, a sample holder 13 positioned atop the thermal interface 12, a cryocooler 22 and a thermal link 30 that provides a connection between the cryocooler 22 and the sample holder 13. A radiation shroud 19 envelopes the upper end of the cryocooler 22, the thermal link 30, the thermal interface 12 and the sample holder 13. The radiation shroud 19 is preferably nested within a vacuum enclosure 18.

The cryocooler 22 has a head section 24 that terminates at a cold tip 26. In this embodiment, a cold tip extension 28 connects the cold tip 26 to the thermal link 30. There are many brands and models of cryocoolers, such as a Sumitomo Cryocooler model DE204, that would be suitable for use in the cryostat 10.

Referring to FIGS. 2A and 2B, the thermal interface 12 includes a top surface 17 to which the sample holder 13 is attached (see FIG. 1). It should be understood that, in other embodiments, the thermal interface 12 could be placed in contact with other types of sample holders, or other devices, such as a digital scanning calorimeter (DSC) or gas chromatograph. Examples of other potential applications include microscopes, X-ray diffraction, porosity measurement devices, vibrating scanning magnetometers, and large temperature range furnaces.

The thermal interface also includes a heating element recess 16, in which a heating element (not shown) is inserted when the cryostat 10 is fully assembled. In this embodiment, the heating element is preferably a cartridge type heating element such as Watlow model E1A-526. A platinum sensor (not shown) is employed to measure temperatures in the thermal interface 12 in the range of 77K to at least 1000K. The thermal interface 12 is sometimes referred to in the art as the “hot end” of the cryostat 10 because it contains the heating element and is the component of the cryostat 10 that is the hottest when the cryostat 10 is operated in high-temperatures.

Referring to FIGS. 2B and 3A, in this embodiment, the thermal link 30 comprises a base 32 and a sleeve 34 having an upper end 38 that is attached to the thermal interface 12. The sleeve 34 also includes a flange 40 that is distal to its upper end 38 and is shaped to fit over an upper end 44 of the base 32. The sleeve 34 preferably maintains a strong physical and thermal connection with the thermal interface 12 and a strong physical connection but low thermal and electrical conductivity with the base 32. This configuration provides strong mechanical integrity for the thermal link 30, while providing low thermal and electrical conductivity between the base 32 and to the thermal interface 12 through the sleeve 34. Providing low electrical conductivity between the base 32 and thermal interface 12 is advantageous in applications where it is desirable to isolate the sample holder 13 (see FIG. 1) from electrical noise generated by the cryocooler 22.

In this embodiment, the flange 40 is secured to the upper end 44 of the base 32 with screws 35 a, 35 b, 35 c (see FIG. 2A). The screws 35 a, 35 b, 35 c are preferably made of a material having low thermal and electrical conductivity, such as a polymer or ceramic material. The screws 35 a, 35 b, 35 c are omitted from FIGS. 2B through 3B, as well as FIGS. 4A and 4B of the second embodiment.

A ceramic sleeve 14 (see FIG. 2A), which covers the thermal interface 12 and a portion of the thermal link 30, may also be provided. The ceramic sleeve 14 is preferably made of a machinable ceramic material. It's primary function is to preserve heat generated by the thermal interface 12, which reduces power requirements at high operating temperatures. In this embodiment, the ceramic sleeve 14 is secured to the sleeve 34.

The thermal link 30 also includes a connector 36 which is preferably secured to the base 32. In this embodiment, the connector 36 is a sapphire rod, which is secured to the base 32 via a friction-fit into a cylindrical recess 42 formed in the upper end 44 of the base 32. Alternatively, the connector 36 could be made of any material having thermal conductivity characteristics that vary inversely with temperature (i.e., thermal conductivity decreases as temperature increases) and, as will be discussed herein, the desired thermal expansion characteristics. Quartz is another example of a suitable material.

The connector 36 is releasably connected to the thermal interface 12. In this embodiment, the thermal interface includes a cylindrical core 39 that extends toward the connector and includes a cylindrical recess 46. The cylindrical recess 46 of the cylindrical core 39 is preferably sized so that the connector 36 can slide easily in and out of the cylindrical recess 46. Optionally, a foil or gasket material (not shown) having high thermal conductivity and a melting point above the maximum operating temperature of the thermal interface 12 could be provided between the connector 36 and each of the recesses 42, 46. Examples of suitable foil or gasket materials include Tantalum, Molybdenum or Tungsten. The sleeve 34 provides a protective casing for the connector and other internal parts of the thermal link 30.

The thermal interface 12, base 32, sleeve 34 and connector 36 are adapted to cause the connector 36 to be firmly seated in the cylindrical recess 46 when the thermal interface 12 is within a low-temperature range and to cause the connector 36 to become disconnected from the thermal interface 12 (in this embodiment, the recess 46) before the thermal interface 12 reaches a critical temperature. Preferably, the critical temperature is a temperature at which the cold tip 26 (see FIG. 1) of the cryocooler 22 is at risk of being damaged. For example, the critical temperature for this embodiment could be in the range of 350-800K and, more preferably, 350K-600K.

In this embodiment, the characteristics discussed in the previous paragraph are enabled by configuring the sleeve 34 so that its total thermal expansion is greater than that total thermal expansion of the thermal interface 12, the base 32 and the connector 36. Stated another way, as the temperature of the thermal link 30 rises, the increase in the length of the sleeve 34 will be greater rate than the sum of (a) the decrease in distance between the recess 42 and recess 46 and (b) the increase on length of the connector 36.

In this embodiment, this is accomplished via the dimensions of the sleeve 34 and by using a material in the sleeve 34 that has a larger coefficient of thermal expansion than the material from which the connector 36 is formed and similar to that of the thermal interface 12 and the base 32. In this embodiment, the thermal interface 12 and base 32 are made of copper, the connector 36 is made of sapphire, and the sleeve 34 is made of type 304 stainless steel. The dimensions of each of these components are also selected to provide a firm physical (and therefore, thermal) connection between the recess 46 and the connector 36 when the thermal interface 12 is within a low-temperature range (e.g., 4K to 300K). Within this low temperature range, relatively high thermal conductivity between the cryocooler 22 and the thermal interface 12 is desirable.

As the temperature of the thermal interface 12 (and therefore the thermal link 30) rises, the sleeve 34 will elongate at a much greater rate than the connector 36 because the coefficient of linear thermal expansion of type 304 stainless steel is greater than that of sapphire. Due to this thermal expansion coefficient differential, the connector 36 will begin to separate from the recess 46 of the cylindrical core 39 as the temperature of the thermal interface 12 approaches the critical temperature and become fully disconnected from the recess 46 at temperatures at or above the critical temperature. Other materials could be used for one or more of the components of the thermal link 30, as long as a sufficient thermal expansion coefficient differential is provided.

Disconnection of the connector 36 from the thermal interface 12 significantly reduces thermal conductivity between the thermal interface 12 and the base 32. This enables the thermal interface 12 to be operated at temperatures well above 800K without damaging the cryocooler 22. In addition, it enables improved temperature recovery because the cold tip 26 remains cooler during high-temperature operation of the cryostat 10.

The thermal link 30 could be used to connect other types of hot and cold interfaces, such as wide temperature range heat treatment devices, microscope sample temperature cycling, and large temperature range furnaces, for example. In addition, multiple thermal links 30 could be assembled in series, particularly in applications where the thermal interface 12 will be operated at very high temperatures.

The graph shown in FIG. 4 shows (a) a representative temperature profile for the cryocooler cold tip of a prior art cryostat having a sapphire connector that is not designed to disconnect from the thermal interface at high temperatures and (b) the temperature profile for the cold tip 26 of a prototype of the cryostat 10 described above. As the graph shows, the temperature of the cold tip of the prior art cryostat rises proportionally with a thermal interface temperature and reaches 350K at a thermal interface temperature of about 800K. Accordingly, the prior art cryostat cannot operate at temperatures above 800K in the thermal interface without damaging the cryocooler. In contrast, the temperature of the cold tip 26 never exceeded 100K while the thermal interface was heated from 100K to 800K. In fact, the temperature of the cold tip 26 actually dropped between thermal interface 12 temperatures of 200K-300K, due to disconnection of the connector 36 in this temperature range.

As shown in FIG. 5, the temperature of the cold tip 26 of the cryostat 10 only reached 100K after the thermal interface 12 was held at nearly 1000K for an extended period of time. Based on testing performed on a prototype of the cryostat 10, it could be adapted to perform uninterrupted temperature cycles with temperatures ranging as high as 1500K and as low as 4K, particularly if multiple thermal links 30 are provided in series. Appropriate materials need to be selected that can achieve 1500K without any damage or degradation in performance.

A second embodiment of the thermal link 130 is shown in FIGS. 6A and 6B. In this embodiment, elements having corresponding elements in the first embodiment are represented by reference numerals increased by factors of 100 (for example, the thermal interface 12 shown in FIGS. 1-3B corresponds to the thermal interface 112 in FIGS. 6A and 6B). Reference numerals for features appearing in both embodiments may be shown in FIGS. 6A and/or 6B without a specific reference in the specification.

The thermal link 130 includes a sapphire disk 150 that is retained against the end of a core 139 by a stainless steel cap 152 having complimentary threads (not shown). The cap 152 also includes an opening 154 which enables the connector 136 to make a firm physical connection with the disk 150 when the thermal interface 112 is operated at low-temperatures and provides a “cleaner” disconnect as the connector 136 disengages from the disk 150 due to differential thermal expansion of the sleeve 134.

Use of the disk 150 and cap 152 potentially extends the service life of the thermal link 130 by enabling the disk 150 to be replaced if necessary due to repeated connection and disconnection of the connector 136. In addition, this design reduces the likelihood that misalignment of the connector 136 would cause a failure and the cap 152 acts as a guide for the connector 136.

A third embodiment of the thermal link 230 is shown in FIG. 7. In this embodiment, elements having corresponding elements in the first embodiment are represented by reference numerals increased by factors of 200 (for example, the thermal interface 12 shown in FIGS. 1-3B corresponds to the thermal interface 212 shown in FIG. 7). Reference numerals for features appearing in the first or second embodiment and the third embodiment may be shown in FIG. 5 without a specific reference in the specification.

In this embodiment, a two-part sleeve 234 is provided. The upper portion 258 of the thermal link 230 is nearly identical in configuration to the sleeve 34 of the first embodiment. The lower portion 260 of the sleeve 234 is positioned between the upper portion 258 and the base 232 and includes an inner portion 262 that sits atop the base 232 and a flange 264 that is connected to the flange 240 of the upper portion 258 with screws 235 a and 235 b. As in the first and second embodiments, the screws 235 a and 235 b are preferably made of a material having relatively low thermal and electrical conductivity, such as a polymer or ceramic material. A very low thermal conductivity spacer 266 is positioned between the lower portion 260 of the sleeve 234 and the base 232 in order to reduce conductive heat load to the base 232. The two-part sleeve 234 design provides reduced thermal load on the base 232 and on the cold tip of the cryocooler (not shown in this embodiment).

While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention. 

1. An apparatus comprising: a thermal interface; a cryocooler; and a thermal link having a thermal connector that provides a first physical link between the cryocooler and the thermal interface when the temperature of the thermal interface is below a critical temperature; wherein the thermal link is adapted to cause the first physical link to be terminated when the temperature of the thermal interface reaches the critical temperature and to prevent reconnection of the first physical link when the temperature of the thermal interface is at a temperature that is above the critical temperature.
 2. The apparatus according to claim 1, wherein the thermal link further comprises: a base that is connected to the cryocooler and the thermal connector; and a first member that provides a second physical link between the cryocooler and the thermal interface when the temperature of the thermal interface is below the critical temperature and when the temperature of the thermal interface is above the critical temperature.
 3. The apparatus according to claim 2, wherein the coefficient of thermal expansion of the first member is larger than the coefficient of thermal expansion of the thermal connector.
 4. The apparatus according to claim 2, wherein the first member comprises a sleeve that surrounds the thermal connector.
 5. The apparatus according to claim 2, wherein thermal expansion of the first member causes the thermal connector to disconnect from the thermal interface when the temperature of the thermal interface is equal to or greater than the critical temperature.
 6. The apparatus according to claim 2, further comprising a spacer positioned between the first member and the base, the spacer having lower thermal conductivity than the first member.
 7. The apparatus according to claim 2, wherein the thermal interface further comprises a disk that is positioned between one end of the thermal connector and the thermal interface and is removably secured to the thermal interface.
 8. The apparatus according to claim 2, wherein the base comprises a first recess and the thermal interface comprises a second recess, one of the first and second recesses being sized to retain the thermal connector therein when the temperature of the thermal link is equal to or greater than the critical temperature and to prevent deflection of the thermal connector.
 9. The apparatus according to claim 1, wherein the thermal conductivity of the thermal connector varies inversely with temperature.
 10. The apparatus according to claim 1, wherein the critical temperature is in the range of 350 degrees Kelvin to 800 degrees Kelvin.
 11. The apparatus according to claim 1, wherein the thermal interface is in contact with one from the group of: a cryostat, a digital scanning calorimeter, a gas chromatograph, a microscope, an X-ray diffractor, a porosity measurement device, and a vibrating scanning magnetometer.
 12. The apparatus according to claim 1, further comprising a sleeve that covers at least a portion of the thermal link and at least a portion of the thermal interface.
 13. A method comprising: (a) cooling a thermal interface to a first temperature using a cryocooler, the thermal interface having a heating element and being connected to a cryocooler through a thermal link having first and second portions; (b) heating the thermal interface with the heating element from the first temperature to a second temperature, the second temperature being greater than the first temperature; and (c) during the heating step, causing a first portion of the thermal link to physically disconnect from the cryocooler or the thermal interface when the thermal link reaches a critical temperature, the critical temperature being between the first and second temperatures.
 14. The method according to claim 13, wherein the heating step further comprises heating the thermal interface with the heating element from the first temperature to a second temperature, the second temperature being greater than 800 degrees Kelvin.
 15. The method according to claim 14, further comprising: (d) maintaining a cold tip of the cryocooler at a temperature no greater than 350 degrees Kelvin during the heating step.
 16. The method according to claim 13, wherein the heating step further comprises heating the thermal interface with the heating element from the first temperature to a second temperature, the second temperature being greater than 1000 degrees Kelvin.
 17. The method according to claim 16, further comprising: (e) maintaining a cold tip of the cryocooler at a temperature no greater than 350 degrees Kelvin during the heating step.
 18. The method according to claim 13, wherein step (c) further comprises maintaining a physical link between the thermal interface and the cryocooler via a second portion of the thermal link when the thermal link is equal to or greater than the critical temperature.
 19. The method according to claim 13, wherein step (c) further comprises causing a first portion of the thermal link to physically disconnect from the cryocooler or the thermal interface due to the second portion of the thermal link having a greater coefficient of thermal expansion than the first portion.
 20. The method according to claim 13, wherein step (c) further comprises cooling the sample to a first temperature using a cryocooler, the first temperature being less than 150 degrees Kelvin. 